Retarded IsoscalarM2Strength Observed in the ReactionO16(ppol, ppol′)O16(2−, 8.88 MeV)atEp=40MeV

Measurement of the spin-flip probability in the reaction ${}_{}{}^{10}{}_{}{}^{}\mathrm{O}(p_{\mathrm{pol}}, {p_{\mathrm{pol}}}^{\prime }){}_{}{}^{16}{}_{}{}^{}\mathrm{O}(2^{-}, 8.88 \mathrm{MeV})$ at $E_p=40\mathrm{}$ MeV yields values that are significantly smaller than those predicted by microscopic distorted-wave Born-approximation calculations. This is interpreted as direct evidence for the dominance of multistep processes in this reaction. Analysis of the data in terms of the spin-flip cross section (spin-flip probability times the differential cross section) yields an isoscalar $M2\mathrm{}$ strength for the $2^{-}$ which is significantly smaller than that predicted by random-phase-approximation wave functions.